The invention relates to a cooling device for a shroud of a gas turbine blade, having a cooling system inside the shroud, which cooling system has, radially to the gas turbine blade, cooling channels that are open on one side, and which is closed off with a cover plate.
Because of the high combustion temperatures in powerful, modern gas turbine systems, the gas turbine components exposed directly or indirectly to the hot gases created during the combustion process are cooled actively. In particular, the gas turbine blades directly exposed to the hot gases are provided with internal cooling channels, through which the cooling air is guided radially from the blade root through the blade hub in the direction of the blade shroud in order to cool, if possible, all areas of a gas turbine blade effectively from the inside. The efficiency of such cooling measures plays a decisive role for the maximum thermal stressability and, ultimately, the maximum life span of a gas turbine blade. Since the cooling air is fed into the cooling channel system from the sides of the blade root, the cooling effect of the cooling air is reduced as a result of the increasing heating in the radial direction towards the blade tip or the shroud area. However, those areas of a turbine blade extending radially are subject to the greatest thermal and, in particular, mechanical loads, especially rotating blades, which are subject, because of their rotation, to significant centrifugal forces.
A gas turbine blade provided with a shroud or outer platform is provided for cooling purposes inside the shroud with a hollow channel system that comprises superficial structures deflecting the cooling air flow in order to distribute and guide the cooling air. For manufacturing reasons of a gas turbine blade that can be produced by way of a casting process, the shroud area provided with the flow channels is constructed radially open on one side. With the help of a cover plate, the flow area for the cooling air is closed off inside the shroud by joining the cover plate flush with the radially oriented topside of the shroud. In general, this is achieved by high-temperature soldering. Such a joint connection between a shroud 1 and a cover plate 2 is shown in a partial cross-section illustration according to FIG. 2. The cover plate 2 is joined in a fixed manner by way of a high-temperature soldering joint 3 with the circumferential edge of the shroud 1. The cooling channels 4 that are constructed radially open on one side and are limited by the cooling air guide surfaces 5 are closed off radially in a gas-tight manner by the cover plate 2. The partial cross-section illustration according to FIG. 2 does not show the inflow and outflow cooling channels oriented radially inside the turbine blade, through which cooling air is fed into and discharged from the shroud area, whereby the cooling air in the cooling channels 4 cools the turbine blade tip. As already mentioned in the beginning, the cooling air flowing through the shroud area is only able to cool with a relatively low efficiency because of the heating present in the radial direction in the gas turbine blade, so that measures must be sought through which the cooling effect can be increased especially in the shroud area by way of the cooling air.
The invention is based on the objective of further developing, with respect to an improved cooling effect, a cooling device for a shroud of a gas turbine blade with a cooling channel system provided in the shroud, said cooling channel system having, radially to the gas turbine blade, cooling channels that are open on one side, and which is closed off with a cover plate, so that the thermally and mechanically greatly stressed shroud area of a gas turbine blade can be better cooled. The measures to be implemented should require very little additional constructive and technical expenditure, but at the same time may cause a clearly better cooling effect than is the case for the shroud cooling measures known so far.
According to the invention, a cooling device is further developed in such a way that an impact cooling plate is provided between the cover plate and the cooling channels that are open on one side in such a way that the impact cooling plate rests on the cooling channels that are open on one side, is pressed against them by force, and encloses a space with the cover plate.
By providing an impact air cooling plate in the flow path of the cooling channel system in the cover plate, cooling air passes through narrowed flow openings inside the impact air cooling plate, so that the cooling air is locally accelerated and, in the form of an impact air cooling jet, impacts a cooling channel wall that is located opposite from the passage opening in the flow direction of the inside of the impact air cooling plate. As a result of the accelerating effect of the impact air cooling plate on the cooling air flow, the heat transfer coefficient between the gas turbine blade and the cooling air flow is locally increased, and the cooling effect of the cooling air flow is improved.
By providing a distance between the impact cooling plate and the cover plate, it is possible to feed a cooling air flow by way of forced guidance through a corresponding cooling channel into the space enclosed between cover plate and impact cooling plate, so that the cooling air, as a result of the overpressure occurring in the space, forces the cooling air to pass through the plurality of flow openings provided in the impact cooling plate. On the other hand, the cooling airflow is able to flow by way of an alternative, suitable cooling channel arrangement also through the impact cooling plate in opposite direction, so that the cover plate is acted on with impact air in a targeted manner.
In order to ensure, on the one hand, an, if possible, gas-tight resting of the impact cooling plate on the shroud""s cooling channel system that is constructed as open on one side, and, on the other hand, to ensure a positioning of the impact cooling plate, if possible, free of bending tensions between the cover plate and the shroud, the impact cooling plate is pressed by force against the cooling channels that are open on one side. The pressing is done with the help of at least one spacer element, which is constructed so as to be preferably elastically or plastically deformable and is provided between the cover plate and impact cooling plate, preferably in the edge area of the impact cooling plate.
The cover plate itself, as already mentioned initially, is joined in a fixed manner with the shroud using standard joining techniques, for example high-temperature soldering, whereby the combustor pulsations generate the pressing force onto the impact cooling plate necessary for the force-derived connection. In order to avoid high pressing forces and to ensure the possibility of a largely free thermal expansion for the impact cooling plate, the spacer element is constructed so as to be elastically or plastically deformable, so that in the case of exceeding a maximum permissible joining force between the cover plate and the impact cooling plate, a deformation of the spacer element occurs.
In a simplest embodiment, the impact cooling plate is joined only along its circumferential edge between the cover plate and shroud with the help of the aforementioned spacer element, which is constructed in the form of a sealing lip extending near the circumferential edge of the impact cooling plate. In order to avoid possible superficial warping within the impact cooling plate, either additional spacer elements can be placed between the cover plate and impact cooling plate, or the impact cooling plate should be constructed with correspondingly designed elevations and/or recesses that come into contact with the cover plate and/or the shroud, and which fixate the impact cooling plate by force yet without tension.
In order to avoid cooling air leaks at those points where the impact cooling plate is pressed by force against the shroud or, on the other side, against the cover plate, additional sealing elements are used, each of which can be positioned between the impact cooling plate and the shroud or cover plate.
A spring-like element can furthermore be used advantageously to press the impact cooling plate by way of the surface of the cover plate against the shroud""s cooling system that is constructed as open on one side. This gives the impact cooling plate a surface pressing effect with respect to the shroud, so that the impact cooling plate can be pressed in an almost gas-tight manner against the structure of the cooling channels.